return figure objects in visualization functions

It will be useful for the plot functions in `acoustic_utils` to return a figure object which could be manipulated and saved as the user wished in follow up steps. Example of how that was needed in this github action workflow:

https://github.com/orcasound/orca-action-workflow/blob/f05ef1b69114cf12ec23c3d9768cd9f0ff63b6d2/noise_processing.py#L61